This invention relates to a current-sense resistor, and more particularly to a current-sense resistive lug used as a connector in a hybrid power module.
Electrical and electronic applications frequently use modules for many purposes. A module comprises several electronic components such as transistors connected in a standard arrangement in one package. Monolithic modules, also known as integrated circuits, consist of one piece of semiconductor material, typically silicon. Hybrid, or multi-chip, modules consist of two or more pieces of semiconductor material connected to one substrate.
Modules have the principal advantage of saving cost and speeding manufacturing compared with hand-assembled components. In addition, all the heat from a module is dissipated in one surface. This may aid in the design of the device that incorporates the module.
One use of modules is in control circuits for electric motors. These modules may be used as inverters to convert DC to AC to power a motor. It is often useful to measure the current between one or more transistor switches within the module and a resistive or inductive load. The load may be a motor winding connected to the transistor switches. The modules may be used instead to convert AC to DC, that is, as synchronous rectifiers. In that application, it may be useful to measure the current at the input to the transistor switches.
A current measurement may be used to protect the switch or the load from damage. Secondly, the measurement may be used to control the torque of the motor. Thirdly, the measurement may be used to control the positional state of the motor, for example, the angle of rotation. For these and other reasons, it is desirable to measure the current. The measured current may be DC or AC, and the AC current may be sinusoidal, square wave, or other waveforms.
In several prior approaches, devices external to the module, or additional devices within the module, were used to measure current. The laws of physics allow current to be measured in only a few ways. For example, Hall effect transducers are known in the art. Secondly, current transformers are well known in the art. Thirdly, additional current-sense resistor shunts within or external to the module are well known. Finally, it is also known to use transistors having Kelvin connections. Shunt resistors or Kelvin connections carry a fraction of the current to be measured.
In an application in which power transistors are mounted on a substrate to form a switch in a module, a sense resistor might be made part of the substrate. The resistor might also be mounted at right angles to the substrate. However, all the designs described herein before require space, add weight, generate heat, and increase the expense or difficulty of manufacturing the hardware.
It is known to use a conducting lug, for example, one made of tinned copper, to connect cables or bus bars to transistor switches, or to other electronic components. A lug is simply a projection on a metal part that is used as a connection. These prior lugs could not be used to sense current because the voltage drop across them would be near zero, and would be lost in the noise.
Accordingly, an object of the present invention is to provide a current-sense apparatus that will fit within a power module.
Another object of the invention is to provide a current-sense lug with relatively small resistance that allows current to be determined by measuring the voltage drop across the lug.
Still another object of the invention is that the voltage drop across the lug is sufficient to be above the electronic noise, and yet the resistance of the lug is low enough so that the heat produced is small compared with the heat generated by the transistor switches.
Yet another object of the present invention is that the resistance of the lug varies by only a relatively small amount with temperature.
A further object of the present invention is that the material comprising the lug is of a type and strength to be readily manufactured.
Another object of the invention is that the lug has sufficient strength to help fasten a cable end to transistor switches. The term xe2x80x9ccablexe2x80x9d as used herein should be understood to mean a cable, a bus bar, or other similar devices for conducting electricity.
Yet another object of the invention is that the lug carries the entire current to be measured.
Still another object of the invention is that the lug is used not only to secure the cable, but also to sense current, so that the current-sense function takes up no additional space in the module.
A major step in the invention is the recognition that the existing means to connect a cable to a transistor switch can also be used to sense current if the resistance value of the connector lug is of a certain, predetermined value.
According to the invention, a power hybrid module has at least one transistor connected as a switch, a cable connecting the output point of the switch to an inductive or resistive load such as a motor winding, and a lug that fastens and electrically connects the cable to the output point of the switch. The lug has a predetermined, known resistance value that allows the current in the cable to be determined by measuring the voltage drop across the lug, the lug resistance varies minimally with temperature, and the lug material has sufficient strength to be manufactured and to help fasten the cable.
The invention has utility because it provides a miniature current-sense apparatus that fits within a power module. No additional parts are used, no additional space is needed, no weight is added, and relatively little heat is dissipated. The material comprising the lug costs little compared with the remainder of the module. The material is of a type and strength to be readily manufactured. Current is determined by measuring the voltage drop across the lug, which electrically connects and physically attaches the cable to the transistor switch. Unlike prior shunt resistors or Kelvin connections, the lug carries the entire current. The lug is used, therefore, not only to attach the cable, but also to sense current; hence, the current-sense function takes up no additional space in the module. The voltage drop across the lug is sufficient to be above the electronic noise. The heat produced by the lug, however, is small compared with the heat generated by the transistor switches. The current-sense process is nearly linear because the resistance of the lug varies by only a relatively small amount with temperature.
The above and other objects, features, and advantages of this invention will become apparent when the following description is read in conjunction with the accompanying drawings.